In standardized digital wireless systems like digital cellular radio networks there is a standardized form for each message that a transmitting device is allowed to transmit over the wireless interface to the receiving device. Due to the limited number of available transmission frequencies and the requirements for minimized delays each message should be as compact as possible. This has resulted in difficulties when extensions and additions to the existing standards are made afterwards.
In digital wireless systems the contents of all messages are basically sequences of bits with a limited sequence length. The specifications usually reserve some otherwise unspecified bit combinations for future additions and extensions, but it may happen that in the future so many additions and extensions are made that these simply run out. In such a situation it is usually required that a completely new kind of a message is specified, which has serious implications in the form of required upgrading of system hardware and software.
As an example we will examine the known packet access burst format which is defined in the specifications of the GSM system (Global System for Mobile telecommunications) for the use of mobile stations when they want to request resources for a dedicated packet data channel. The definition of a packet access burst covers 157 consecutive bits according to Table I.
TABLE IFieldBit no.lengthField contentsDefinition 0–78extended tail bits0, 0, 1, 1, 1, 0, 1, 0 8–4841synchronization0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1, 1,sequence1, 1, 1, 1, 1, 0, 0, 1, 1, 0, 0, 1,1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1, 1,1, 1, 0, 0, 0,49–8436encrypted bits11 information bits, 6 paritybits and a base stationidentifier; all coded together85–873tail bits0, 0, 088–15668.25extended guard periodperiod of no transmissionat all
The actual information content of the packet access request must be represented by the 11 information bits mentioned on the middle row of Table I. It is clear that the amount of information that can be represented with only 11 bits is quite limited. Table II shows the definitions which are currently given to the contents of the field of 11 information bits.
TABLE IIBitsPacket1110987654321Channel Access0mmmmmpprrrOne Phase AccessRequest (OPAR)100nnnpprrrShort AccessRequest (SAR)110000pprrrTwo Phase AccessRequest (TPAR)110001pprrrPage Response110010pprrrCell Update110011pprrrMobility Man-agement Procedure110100rrrrrSingle Block w/oTBF establishmentAll othersReserved
In the table a letter m represents a bit that is a part of a MS multislot class indicator (there are 29 classes, so 5 bits are required), a letter p represents a bit that is a part of a radio priority indicator (4 priorities, 2 bits), a letter n represents a bit that is a part of an indicator revealing the number of RLC (Radio Link Control) data blocks requested during a mobile-originated TBF (Temporary Block Flow) (max. 8 blocks, 3 bits) and a letter r represents a bit that is a part of a random reference in an otherwise unformatted field.
As a specific example, despite the relatively large number of “Reserved” bit combinations there appears to be no suitable and unspecified bit combinations that a mobile station could use to separately indicated its capability of using either the basic GPRS (General Packet Radio System), the EGPRS (Extended GPRS) with eight-level phase shift keying (8PSK) as the modulation method or the EGPRS without 8PSK as the modulation method, when the mobile station is making a One Phase Access Request (OPAR), a Short Access Request (SAR) or a Two Phase Access Request (TPAR). The possibility of even further similar needs is also foreseeable.
A proposal has been made to abandon the rule of having only one allowed form for the synchronization sequence (also known as the training sequence) and to use the selected form of the synchronization/training sequence as an indicator of a certain piece of information. However, this known solution has the drawback that it requires some complicated and potentially expensive changes to be made in some older devices existing in the networks that are to be used to provide the packet data services. Another proposal is to modulate the message to be transmitted with one of a set of alternative modulation methods so that the selection of the modulation method would indicate the modulation and demodulation capabilities of the device transmitting the message. This method is only applicable when there are several modulation methods to choose from, and it rules out the advantageous proposal of using always the simplest and robustest modulation method to modulate important messages like access requests.